1. Field of the Invention
This invention relates to films and sheets formed of blends of polyethylene terephthalate and polycarbonate resins useful as cook-in-trays.
2. Discussion of the Prior Art
Frozen foods, which can be cooked in the tray in which they are packaged, are a standard commercial item. These so-called "convenience" foods are usually packaged in a tray or similar structure formed of aluminum and are intended for home as well as industrial uses. Such trays, however, are somewhat aesthetically unpleasing. Additionally, the price of aluminum has been increasing to very high levels. Consequently, efforts have been expended to find a substitute for aluminum as the material used in forming cook-in-trays and the like. Recently there has been marketed polysulfone trays which are generally thermoformed from a sheet of polysulfone resin. Although material costs and resultant physical properties compare favorably with aluminum trays, relatively long cycle times are required to produce such trays on conventional thermoforming machines.
It is known that various thermoplastic resins can be blended to generally take advantage of the separate properties of each resin. For example, U.S. Pat. No. 3,218,372 issued in 1965 to Okamura et al discloses a molding material formed from a blend of a polyalkylene terephthalate (e.g. polyethylene terephthalate) having an intrinsic viscosity of 0.5 - 0.85 and a polycarbonate having an intrinsic viscosity of 0.46 - 1.2. Blend ratios disclosed in said patent range, in parts by weight, from 95 - 5 polycarbonate and from 5 - 95 polyalkylene terephthalate. It is therein disclosed that compositions containing 95 - 70 polycarbonate and 5 - 30 polyalkylene terephthalate possess low melt viscosities compared with polycarbonate alone so as to facilitate molding operations, whereas compositions containing 5 - 70 parts polycarbonate and 95 - 30 parts polyalkylene terephthalate have enhanced hardness and tensile strength over polyalkylene terephthalate alone.